The Permo-Triassic Formation of Khuff in offshore Abu Dhabi has shown a considerable potential as a tight gas reservoir, but the complex mineralogy and the heterogeneity has made formation evaluation challenging for this reservoir, particularly in respect to saturation. Complexities with log interpretation in these tight formations have called for the use the advanced measurements to help interpret more basic logs in a simple but accurate and consistent manner.
Deriving porosity from density requires the knowledge of matrix density. In complex Khuff reservoir, with a varying mix of dolomite and anhydrite with some calcite, it is impossible to put a constant matrix density. Photoelectric factor is not enough to resolve the mineralogy and advanced spectroscopy measurement is mandatory to provide matrix properties such as grain density, neutron (thermal and epithermal) and sigma (capture cross-section). These properties are used to compensate the formation density, neutron and sigma for the matrix effect, focusing any residual separation between the corrected porosities onto fluid effects.
Matrix-corrected density, neutron and sigma porosity curves can be used effectively to qualitatively interpret presence of gas, which is a big challenge in these tight gas reservoirs and this approach is demonstrated on multiple datasets. Since this is being applied on wireline logs with a depth of investigation shallower than 10-in, uncertainties remain concerning the deep, uninvaded zone water saturation. The deep resistivity measurement is still the only access to deep saturation. To quantify the water saturation from resistivity, besides the knowledge of formation water salinity, the other key information is the Archie parameter m (cementation exponent), which is directly dependent on the texture of the rock. Complex diagenesis on the varying mineralogy of the Khuff has created complex water-phase tortuosity, which needs to be considered. Multi-frequency dielectric measurement in addition to giving a shallow zone water saturation, also provides a water-phase tortuosity parameter, which is then used to improve deep water saturation computation.
When facing a complex reservoir, the temptation is high to jump to complex evaluation workflows, neglecting simple, quick-look-type approaches. However, advanced measurements, such as spectroscopy and dielectric dispersion empower this type of simple approach, enabling efficient controls and insights on the evaluation, as will be shown through the examples in this paper. Final evaluation still requires a fully integrated analysis, but it will benefit from the lesson learnt and insights from the quick-look.